Broadband reflecting mirror

ABSTRACT

To provide a broadband reflecting mirror having high reflectance within a wavelength band of 400 nm to 2500 nm and having excellent thermal resistance and damage resistance. A broadband reflecting mirror  1  for reflecting light within a wavelength band of 400 nm to 2500 nm includes: a first reflective layered coating  3  for reflecting light in a short wavelength side of the wavelength band of 400 nm to 2500 nm, the first reflective layered coating including first high-refractive index material and first low-refractive index material layers alternately stacked one on another; and a second reflective layered coating  4  for reflecting light in a long wavelength side of the wavelength band of 400 nm to 2500 nm, the second reflective layered coating including second high-refractive index material and second low-refractive index material layers alternately stacked one on another, wherein the first reflective layered coating  3  is disposed on the light-incident side of the broadband reflecting mirror and the second reflective layered coating  4  is disposed at a position where light having passed through the first reflective layered coating  3  can be reflected, and wherein the first high-refractive index material layer is formed of at least one material selected from the group consisting of niobium oxide, titanium oxide, zirconium oxide, tantalum oxide, hafnium oxide, silicon nitride, yttrium oxide and indium tin oxide, the first low-refractive index material layer is formed of at least one material selected from the group consisting of silicon oxide and magnesium fluoride, the second high-refractive index material layer is formed of at least one material selected from the group consisting of silicon and germanium, and the second low-refractive index material layer is formed of at least one material selected from the group consisting of silicon oxide and magnesium fluoride.

TECHNICAL FIELD

This invention relates to broadband reflecting mirrors for reflectinglight within a wavelength band of 400 nm to 2500 nm.

BACKGROUND ART

Attention has recently been focused on apparatus and systems utilizingheat energy obtained by collecting sunlight, and development andpractical utilization of solar collectors and solar concentratingsystems are underway.

For example, studies have been conducted on solar power generationsystems in which sunlight is collected, the heat energy therefrom isused to generate high-temperature and high-pressure steam, and the steamis used to drive a turbine or the like.

There is known a system, as a solar concentrating system, in which aplurality of heliostats are placed on the ground and light beamsreflected from the heliostats are reflected by a light-collectingreflecting mirror to collect the sunlight to a collector (see forexample Non-Patent Literature 1).

In order to use heat energy from sunlight, it is necessary to usevisible range and infrared range of light included in sunlight andhaving high heat energy, and a broadband reflecting mirror is thereforerequired for reflecting and collecting visible range and infrared rangeof light.

Generally, reflecting mirrors are produced by coating a thin metal film,such as aluminum or silver, on a transparent substrate, such as glass.However, reflecting mirrors coated with such a thin metal film sufferfrom the problem of being poor in thermal resistance and weatherresistance, because the thin metal films on their surfaces are likely tobe oxidized by the environmental atmosphere.

To solve the above problem, Patent Literature 1 proposes a reflectingmirror for reflecting sunlight, wherein a metallic reflective coating isprovided over a transparent substrate and an inorganic transparentprotective film is provided on the metallic reflective coating.

Likewise, Patent Literature 2 proposes a reflective heat collectingplate in which a transparent protective film of inorganic material isprovided on a reflective metal-deposited coating.

The inventor conducted studies on a reflecting mirror for reflectinglight within a wavelength band of 400 nm to 2500 nm as a reflectingmirror for collecting sunlight, and consequently has found that thereflecting mirror provided with a metallic reflective coating for lightreflection suffers from the problem of having low reflectance,particularly in a visible light range.

Meanwhile, there is known a dielectric mirror, as a reflecting mirror,in which high-refractive index material layers, such as niobium oxide,and low-refractive index material layers, such as silicon oxide, arealternately stacked to reflect sunlight using light interference. If adielectric mirror is used to reflect light within a wavelength band of400 nm to 2500 nm, there arises a problem in that the number of layersstacked must be considerably large.

In addition, there also arises a problem in that as the number of layersstacked increases, the reflecting mirror becomes likely to be warped bythe stress in stacking the layers.

CITATION LIST Patent Literature

-   Patent Literature 1: Published Japanese Patent Application No.    S57-4003-   Patent Literature 2: Examined Japanese Patent Application    Publication No. S62-57904

Non-Patent Literature

-   Non-Patent Literature 1: E. Epstein, A. Segaland and A. Yogev, “A    molten salt system with a ground base-integrated solar receiver    storage tank.” J. Phys. IV France 9, 95-104 (1999)

SUMMARY OF INVENTION Technical Problem

An object of the present invention is to provide a broadband reflectingmirror having high reflectance within a wavelength band of 400 nm to2500 nm and having excellent thermal resistance and damage resistance.

Solution to Problem

The present invention is a broadband reflecting mirror for reflectinglight within a wavelength band of 400 nm to 2500 nm, the broadbandreflecting mirror including: a first reflective layered coating forreflecting light in a short wavelength side of the wavelength band of400 nm to 2500 nm, the first reflective layered coating including firsthigh-refractive index material and first low-refractive index materiallayers alternately stacked one on another; and a second reflectivelayered coating for reflecting light in a long wavelength side of thewavelength band of 400 nm to 2500 nm, the second reflective layeredcoating including second high-refractive index material and secondlow-refractive index material layers alternately stacked one on another,wherein the first reflective layered coating is disposed on thelight-incident side of the broadband reflecting mirror and the secondreflective layered coating is disposed at a position where light havingpassed through the first reflective layered coating can be reflected,and wherein the first high-refractive index material layer is formed ofat least one material selected from the group consisting of niobiumoxide, titanium oxide, zirconium oxide, tantalum oxide, hafnium oxide,silicon nitride, yttrium oxide and indium tin oxide, the firstlow-refractive index material layer is formed of at least one materialselected from the group consisting of silicon oxide and magnesiumfluoride, the second high-refractive index material layer is formed ofat least one material selected from the group consisting of silicon andgermanium, and the second low-refractive index material layer is formedof at least one material selected from the group consisting of siliconoxide and magnesium fluoride.

In the present invention, there are provided a first reflective layeredcoating for reflecting light in a short wavelength side of a wavelengthband of 400 nm to 2500 nm and a second reflective layered coating forreflecting light in a long wavelength side of the wavelength band of 400nm to 2500 nm, wherein the first reflective layered coating is formed byalternately stacking first high-refractive index material and firstlow-refractive index material layers, and the second reflective layeredcoating is formed by alternately stacking second high-refractive indexmaterial and second low-refractive index material layers.

In addition, the first high-refractive index material layer is formed ofat least one material selected from the group consisting of niobiumoxide, titanium oxide, zirconium oxide, tantalum oxide, hafnium oxide,silicon nitride, yttrium oxide and indium tin oxide, the firstlow-refractive index material layer is formed of at least one materialselected from the group consisting of silicon oxide and magnesiumfluoride, the second high-refractive index material layer is formed ofat least one material selected from the group consisting of silicon andgermanium, and the second low-refractive index material layer is formedof at least one material selected from the group consisting of siliconoxide and magnesium fluoride.

In the present invention, at least two different types of reflectivelayered coatings are provided which include the first reflective layeredcoating for reflecting light in the short wavelength side of the abovewavelength band and the second reflective layered coating for reflectinglight in the long wavelength side thereof, whereby the total number oflayers stacked in the entire reflecting mirror can be reduced.Therefore, the manufacturing process can be simplified, resulting inefficient production.

In addition, since the number of layers stacked can be reduced, thewarpage due to stress produced in depositing thin layers can be reduced.

Although silicon or germanium, both of which are metals, is used as eachof high-refractive index material layers, the second low-refractiveindex material layers with which the above thin high-refractive indexmaterial layers alternate are made of silicon oxide or magnesiumfluoride. Therefore, if a layer of silicon oxide or magnesium fluorideis disposed as the outermost layer, the reflecting mirror can be givenhigh thermal resistance and high damage resistance.

In making the second high-refractive index material layer from siliconin the present invention, because silicon has high transmittance withina range of wavelengths longer than 1200 nm, it is preferable that thewavelength band of the short wavelength side to be reflected by thefirst reflective layered coating should be set to be from 400 nm to 1200nm and the wavelength band of the long wavelength side to be reflectedby the second reflective layered coating should be set to be from 1200nm to 2500 nm.

In making the second high-refractive index material layer fromgermanium, because germanium has high transmittance within a range ofwavelengths longer than 2000 nm, it is preferable that the wavelengthband of the short wavelength side to be reflected by the firstreflective layered coating should be set to be from 400 nm to 2000 nmand the wavelength band of the long wavelength side to be reflected bythe second reflective layered coating should be set to be from 2000 nmto 2500 nm.

In the present invention, the first reflective layered coating isdisposed on the light-incident side of the broadband reflecting mirror,and the second reflective layered coating is disposed at a positionwhere light having passed through the first reflective layered coatingcan be reflected. Since the first reflective layered coating and thesecond reflective layered coating are arranged in this manner, thebroadband reflecting mirror can achieve high reflectance. If the secondreflective layered coating were disposed on the light-incident side ofthe reflecting mirror, light within a wavelength band to be reflected bythe first reflective layered coating would be absorbed by the secondreflective layered coating, whereby the reflecting mirror could notachieve high reflectance as a broadband reflecting mirror.

In the present invention, the first reflective layered coating and thesecond reflective layered coating are preferably provided on atransparent substrate. Since these coatings are provided on atransparent substrate, the substrate after sequential stacking of thinlayers thereon can be used as a broadband reflecting mirror as it is.Examples of the transparent substrate include a glass substrate, asapphire substrate and a resin substrate.

In a first embodiment according to the present invention, the firstreflective layered coating is disposed on one surface of the transparentsubstrate, while the second reflective layered coating is disposed onthe other surface of the transparent substrate. According to the firstembodiment, since a reflective layered coating is formed on each side ofthe transparent substrate, the stress to be produced in stacking thinlayers can be produced on both sides of the transparent substrate,whereby the stress can be balanced between both sides of the transparentsubstrate. Therefore, the warpage of the reflecting mirror can bereduced. Thus, according to the first embodiment, a substantiallynon-warped, flat broadband reflecting mirror can be produced.

In a second embodiment according to the present invention, the secondreflective layered coating is disposed on top of the transparentsubstrate, and the first reflective layered coating is disposed on topof the second reflective layered coating. Since the first reflectivelayered coating and the second reflective layered coating are arrangedin this manner, light having passed through the first reflective layeredcoating disposed on the light-incident side of the reflecting mirror isallowed to directly enter the second reflective layered coating. Sincethe broadband reflecting mirror allows light to enter the secondreflective layered coating without passing through the transparentsubstrate, it can achieve high reflectance.

In a third embodiment according to the present invention, the firstreflective layered coating is disposed immediately on the transparentsubstrate, and the second reflective layered coating is disposed on thefirst reflective layered coating. According to this arrangement, thetransparent substrate can be disposed facing the outside, whereby thebroadband reflecting mirror can increase the durability, such as damageresistance and chemical resistance.

Since the broadband reflecting mirror of the present invention canreflect light within a wavelength band of 400 nm to 2500 nm, it can besuitably used as a reflecting mirror for utilizing heat energy fromsunlight. The broadband reflecting mirror can be used, for example, as areflecting mirror for each heliostat in a solar concentrating system oras a light-collecting reflecting mirror for collecting light beamsreflected by reflecting mirrors for heliostats.

The first reflective layered coating in the present invention is formedby alternately stacking first high-refractive index material and firstlow-refractive index material layers. The materials that can be used forthe first high-refractive index material layers and the materials thatcan be used for the first low-refractive index material layers are theabove-mentioned materials. For the first high-refractive index materiallayers, the plurality of stacked layers may be formed using the samematerial or using two or more different materials. For example, all ofthe stacked layers serving as the first high-refractive index materiallayers may be made of niobium oxide, or some of the stacked layers maybe made of another high-refractive index material, such as titaniumoxide. Also for the first low-refractive index material layers, theplurality of stacked layers may be formed using the same material orusing different materials. However, from the standpoint of productionefficiency and the like, the use of the same material is preferred.

The number of layers to be stacked in the first reflective layeredcoating is not particularly limited, but is preferably within the rangeof 30 to 200 layers in a combined total of the first high-refractiveindex material layers and the first low-refractive index materiallayers, and more preferably within the range of 70 to 90 layers.Furthermore, the thickness of the first reflective layered coating,i.e., the total thickness of the stacked first high-refractive indexmaterial layers and first low-refractive index material layers, is notparticularly limited but is preferably within the range of 3 to 20 μmand more preferably within the range of 7 to 10 μm.

Also in the second reflective layered coating, each for the secondhigh-refractive index material layers and for the second low-refractiveindex material layers, the same material may be used or two or moredifferent materials may be used. However, as described previously, fromthe standpoint of production efficiency and the like, the use of thesame material each for the second high-refractive index material layersand for the second low-refractive index material layers is preferred.

The number of layers to be stacked in the second reflective layeredcoating is not particularly limited, but is preferably within the rangeof 9 to 50 layers, and more preferably within the range of 15 to 25layers.

The thickness of the second reflective layered coating, i.e., the totalthickness of the stacked second high-refractive index material layersand second low-refractive index material layers, is not particularlylimited but is preferably within the range of 2 to 10 μm and morepreferably within the range of 2 to 6 μm.

The process for forming the first high-refractive index material layer,the first low-refractive index material layer, the secondhigh-refractive index material layer and the second low-refractive indexmaterial layer is not particularly limited, and these layers can beformed by any common thin film formation process. These layers can beformed, for example, by a sputtering process, a deposition process, suchas ion beam deposition, or a CVD process.

The configuration of the first high-refractive index material layers andfirst low-refractive index material layers in the first reflectivelayered coating and the second high-refractive index material layers andsecond low-refractive index material layers in the second reflectivelayered coating, inclusive of the thicknesses of these layers, can bedesigned through simulation. For example, the configuration can bedesigned using a simulation software commercially available from amanufacturer, such as The Essential Macleod Thin Film Center Inc., TFcalc Software Spectra Inc. or Film Star FTG Software Associates.

In the present invention, a metal coating may be provided as a thirdreflective coating at a position where light having passed through thesecond reflective layered coating can be reflected. The provision of ametal coating as a third reflective coating allows the number of layersstacked in the second reflective layered coating to be reduced. Forexample, the number of layers can be within the range of 2 to 10 layers.Thus, the thickness of the second reflective layered coating can also bereduced. For example, the thickness can be within the range of 0.3 to 1μm.

Examples of the metal coating include aluminum (Al), silver (Ag) andgold (Au). From the viewpoint of thermal resistance, aluminum (Al) ispreferably used. The thickness of the metal coating is not particularlylimited but is preferably within the range of 0.03 to 1 μm and morepreferably within the range of 0.05 to 0.25 μm.

The process for forming the third reflective coating is not particularlylimited, and the coating can be formed by any common thin film formationprocess. The coating can be formed, for example, by a deposition processor a sputtering process.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a broadband reflecting mirror can beobtained which has high reflectance within a wavelength band of 400 nmto 2500 nm and has excellent thermal resistance and damage resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a broadband reflecting mirroraccording to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a broadband reflecting mirroraccording to a second embodiment of the present invention.

FIG. 3 is a schematic diagram showing a broadband reflecting mirroraccording to a third embodiment of the present invention.

FIG. 4 is a graph showing the reflectance of a broadband reflectingmirror of Example 1 according to the present invention.

FIG. 5 is a graph showing the reflectance of the broadband reflectingmirror of Example 1 according to the present invention and thereflectances of broadband reflecting mirrors using conventional metalcoatings.

FIG. 6 is a graph showing the reflectances of the broadband reflectingmirror of Example 1 according to the present invention before and afterheat application.

FIG. 7 is a graph showing the reflectances of the reflecting mirrorusing a Ag (silver) coating before and after heat application.

FIG. 8 is a graph showing the reflectances of the reflecting mirrorusing an Al (aluminum) coating before and after heat application.

FIG. 9 is a graph showing the relation between heat application time andreflectance in Example 1 according to the present invention.

FIG. 10 is a schematic diagram showing an example of a solarconcentrating system.

FIG. 11 is a graph showing the reflectances of broadband reflectingmirrors of Examples 6 and 7 according to the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described with reference tospecific examples. However, the present invention is not limited by thefollowing examples.

First Embodiment

FIG. 1 is a schematic cross-sectional view showing a broadbandreflecting mirror 1 according to a first embodiment. In the firstembodiment according to the present invention, a first reflectivelayered coating 3 is disposed on one surface of a transparent substrate2, while a second reflective layered coating 4 is disposed on the othersurface of the transparent substrate 2. Light is irradiated from the topof the figure, enters the first reflective layered coating 3, passesthrough it, then penetrates the transparent substrate 2, and then entersthe second reflective layered coating 4. The first reflective layeredcoating 3 reflects light in a short wavelength side of a wavelength bandof 400 nm to 2500 nm. Therefore, light having passed through the firstreflective layered coating contains a reduced amount of light within theabove wavelength band and consists mostly of light in a long wavelengthside of the wavelength band.

Light having passed through the first reflective layered coatingpenetrates the transparent substrate 2 and then enters the secondreflective layered coating 4. The second reflective layered coatingreflects light in a long wavelength side of the wavelength band of 400nm to 2500 nm. In the present invention, silicon or germanium is used aseach of second high-refractive index material layers of the secondreflective layered coating. The use of silicon or germanium as each ofthe second high-refractive index material layers allows the number ofstacked layers of the second reflective layered coating to be reduced,whereby the total number of stacked layers of the first and secondreflective layered coatings can be significantly reduced.

Since in the first embodiment of the present invention a reflectivelayered coating is provided on each side of the transparent substrate 2,the stress produced in depositing thin layers can be balanced betweenboth sides of the transparent substrate. Therefore, the warpage due tostress produced in depositing thin layers can be reduced, whereby a lesswarped broadband reflecting mirror can be produced.

Second Embodiment

FIG. 2 is a schematic cross-sectional view showing a broadbandreflecting mirror 1 according to a second embodiment of the presentinvention.

As shown in FIG. 2, in the broadband reflecting mirror 1 of thisembodiment, a second reflective layered coating 4 is provided on onesurface of a transparent substrate 2, and a first reflective layeredcoating 3 is provided on top of the second reflective layered coating 4.Light having passed through the first reflective layered coating 3directly enters the second reflective layered coating 4, whereby ahigh-reflectance broadband reflecting mirror without light absorption inthe transparent substrate 2 can be produced.

Third Embodiment

FIG. 3 is a schematic cross-sectional view showing a broadbandreflecting mirror 1 according to a third aspect of the presentinvention.

As shown in FIG. 3, in the broadband reflecting mirror 1 of thisembodiment, a first reflective layered coating 3 is provided on onesurface of a transparent substrate 2, and a second reflective layeredcoating 4 is provided on the first reflective layered coating 3.

According to this embodiment, the transparent substrate 2 can bedisposed facing the outside, whereby a broadband reflecting mirror canbe produced which has excellent durability, such as damage resistanceand chemical resistance.

Examples that follow are those of a broadband reflecting mirror in whichaccording to the first embodiment of the present invention a firstreflective layered coating is disposed on one surface of a transparentsubstrate and a second reflective layered coating is disposed on theother surface of the transparent substrate.

Example 1

This example is an example of a broadband reflecting mirror in which afirst reflective layered coating is provided on one surface of a glasssubstrate and a second reflective layered coating is provided on theother surface of the glass substrate. The glass substrate is a 0.3 mmthick glass substrate having a trade name “OA-10” manufactured by NipponElectric Glass Co., Ltd. A material used as each of firsthigh-refractive index material layers of the first reflective layeredcoating is Nb₂O₅ (niobium pentoxide), and a material used as each offirst low-refractive index material layers thereof is SiO₂ (siliconoxide). The thicknesses of these layers and the coating configurationare shown in TABLE 1. “Layer No.” in TABLE 1 indicates the order of thelayers from the glass substrate side. As shown in TABLE 1, the number oflayers of the first reflective layered coating is 79.

The broadband reflecting mirror of this example is a broadbandreflecting mirror designed to exhibit its highest reflectance when theangle of incidence of light thereon is approximately 20°.

TABLE 1 Angle of Incidence: 20° Layer No. Type Thickness (nm) 1 Nb₂O₅129.96 2 SiO₂ 219.77 3 Nb₂O₅ 140.53 4 SiO₂ 217.44 5 Nb₂O₅ 138.62 6 SiO₂217.3 7 Nb₂O₅ 136.83 8 SiO₂ 197.46 9 Nb₂O₅ 125.34 10 SiO₂ 198.65 11Nb₂O₅ 118.46 12 SiO₂ 199.9 13 Nb₂O₅ 124.7 14 SiO₂ 200.17 15 Nb₂O₅ 125.5216 SiO₂ 178.4 17 Nb₂O₅ 115.29 18 SiO₂ 177.09 19 Nb₂O₅ 109.18 20 SiO₂171.14 21 Nb₂O₅ 102.79 22 SiO₂ 163.03 23 Nb₂O₅ 98.04 24 SiO₂ 151.06 25Nb₂O₅ 98.01 26 SiO₂ 152.7 27 Nb₂O₅ 108.15 28 SiO₂ 153.73 29 Nb₂O₅ 91.4530 SiO₂ 150.35 31 Nb₂O₅ 92.17 32 SiO₂ 134.35 33 Nb₂O₅ 87.96 34 SiO₂136.55 35 Nb₂O₅ 81.04 36 SiO₂ 127.18 37 Nb₂O₅ 77.38 38 SiO₂ 129.75 39Nb₂O₅ 83.57 40 SiO₂ 114.1 41 Nb₂O₅ 69.86 42 SiO₂ 122.19 43 Nb₂O₅ 74.9344 SiO₂ 121.13 45 Nb₂O₅ 73.43 46 SiO₂ 120.02 47 Nb₂O₅ 71.54 48 SiO₂106.34 49 Nb₂O₅ 64.53 50 SiO₂ 107.75 51 Nb₂O₅ 63.63 52 SiO₂ 98.23 53Nb₂O₅ 60.33 54 SiO₂ 90.62 55 Nb₂O₅ 65.35 56 SiO₂ 93.61 57 Nb₂O₅ 51.52 58SiO₂ 89.76 59 Nb₂O₅ 68.28 60 SiO₂ 89.59 61 Nb₂O₅ 50.86 62 SiO₂ 83.07 63Nb₂O₅ 53.37 64 SiO₂ 100.92 65 Nb₂O₅ 48.9 66 SiO₂ 78.7 67 Nb₂O₅ 45.32 68SiO₂ 72.55 69 Nb₂O₅ 45.98 70 SiO₂ 72.16 71 Nb₂O₅ 45.17 72 SiO₂ 73.19 73Nb₂O₅ 43.1 74 SiO₂ 72.64 75 Nb₂O₅ 43.37 76 SiO₂ 67.11 77 Nb₂O₅ 41.08 78SiO₂ 66.64 79 Nb₂O₅ 33.74

In the second reflective layered coating, a material used as each ofsecond high-refractive index material layers is Si (silicon), and amaterial used as each of second low-refractive index material layers isSiO₂ (silicon oxide).

The configuration of the second reflective layered coating is shown inTABLE 2.

In TABLE 2, “Layer No.” indicates the order of the layers from the glasssubstrate side. As shown in TABLE 2, the number of layers of the secondreflective layered coating in this example is 25. Furthermore, as shownin TABLE 2, in order to increase the thermal resistance, a secondlow-refractive index material layer made of SiO₂ is disposed as theoutermost layer.

TABLE 2 Layer No. Type Thickness (nm) 1 SiO₂ 50 2 Si 99.53 3 SiO₂ 247.734 Si 99.53 5 SiO₂ 247.73 6 Si 99.53 7 SiO₂ 247.73 8 Si 99.53 9 SiO₂247.73 10 Si 99.53 11 SiO₂ 317.09 12 Si 155.27 13 SiO₂ 386.46 14 Si155.27 15 SiO₂ 386.46 16 Si 155.27 17 SiO₂ 386.46 18 Si 155.27 19 SiO₂386.46 20 Si 155.27 21 SiO₂ 495.46 22 Si 199.07 23 SiO₂ 495.46 24 Si199.07 25 SiO₂ 100

As described so far, the broadband reflecting mirror of this example hasa first reflective layered coating made of 79 layers on one side of theglass substrate and a second reflective layered coating made of 25layers on the other side of the glass substrate. Thus, the total numberof layers stacked in the entire broadband reflecting mirror is 104.

If the second reflective layered coating were composed of Nb₂O₅ and SiO₂like the first reflective layered coating, its number of layers would be67. Thus, the total number of layers in the entire broadband reflectingmirror would be 146. In contrast, if according to the present inventionsilicon or germanium is used as each second high-refractive indexmaterial layer of the second reflective layered coating, the number oflayers to be stacked can be significantly reduced.

FIG. 4 is a graph showing the reflectance of this example at wavelengthsof 400 nm to 2500 nm. As is evident from FIG. 4, the broadbandreflecting mirror of this example has achieved high reflectance over theentire wavelength band of 400 nm to 2500 nm.

FIG. 5 is a graph showing the reflectances of comparative reflectingmirrors produced by forming a Ag coating, an Al coating and a Au coatingas their respective metal coatings on their glass substrates made of thesame material as used in Example 1, together with the reflectance ofthis example.

In FIG. 5, an enlarged graph is also given in which a zone ofreflectance of 80 to 100% in the above graph is shown in an enlargedmanner. Note that in the enlarged graph the scale of wavelength on theabscissa axis represents that in the graph of reflectance of 0 to 100%.

As shown in FIG. 5, it can be seen that this example (Inventive Example)exhibited high reflectance of near 100% over the entire wavelength bandof 400 nm to 2500 nm, but on the other hand the reflecting mirrorsformed using a Ag coating, an Al coating and a Au coating, respectively,reduced their reflectance in a visible light range.

FIG. 6 is a graph showing changes in reflectance of the broadbandreflecting mirror of this example between before and after beingsubjected to a heat application test. The condition of the heatapplication test was to apply heat at 300° C. for 264 hours.

As shown in FIG. 6, no significant change is found in the reflectancegraph between before and after the heat application, and the reflectancecurves in the graph are substantially superposed. Therefore, it can beunderstood that the reflecting mirror of this example has excellentthermal resistance.

FIG. 7 is a graph showing the reflectances of the reflecting mirrorusing a Ag coating before and after heat application. The condition ofthe heat application was to apply heat at 300° C. for an hour.

As shown in FIG. 7, it can be seen that after the heat application thereflecting mirror reduced the reflectance, particularly in a visiblelight range. Therefore, the reflective mirror using a Ag coating,although having high reflectance as compared to the reflecting mirrorsusing the other types of metal coatings as shown in FIG. 5, can beunderstood to be poor in thermal resistance.

FIG. 8 is a graph showing the reflectances of the reflecting mirrorusing an Al coating before and after heat application.

As shown in FIG. 8, in the reflecting mirror using an Al coating, nosignificant change is found between before and after the heatapplication, and the reflectance curves in the graph are superposed.Therefore, the reflecting mirror using an Al coating has good thermalresistance. However, as shown in FIG. 5, the reflecting mirror has poorreflectance.

FIG. 9 is a graph showing the thermal resistance of the broadbandreflecting mirror of this example. Measured in this case were changes inaverage reflectance at wavelengths of 400 nm to 2500 nm with heatapplication time. As shown in FIG. 9, it can be seen that the averagereflectance in this example was not substantially changed even if heatapplication at 300° C. was continued for 264 hours.

Example 2

A broadband reflecting mirror of this example is a broadband reflectingmirror exhibiting its highest reflectance when the angle of incidencethereon is approximately 16°.

Like Example 1, Nb₂O₅ is used as each of first high-refractive indexmaterial layers, SiO₂ is used as each of first low-refractive indexmaterial layers. TABLE 3 shows the configuration of the first reflectivelayered coating.

TABLE 3 Angle of Incidence: 16° Layer No. Type Thickness (nm) 1 Nb₂O₅128.22 2 SiO₂ 221.29 3 Nb₂O₅ 142.67 4 SiO₂ 217.6 5 Nb₂O₅ 133.58 6 SiO₂213.26 7 Nb₂O₅ 129.62 8 SiO₂ 194.52 9 Nb₂O₅ 125.57 10 SiO₂ 199.75 11Nb₂O₅ 121.93 12 SiO₂ 197.05 13 Nb₂O₅ 123.56 14 SiO₂ 202.89 15 Nb₂O₅127.36 16 SiO₂ 178.84 17 Nb₂O₅ 117.93 18 SiO₂ 179.4 19 Nb₂O₅ 110.85 20SiO₂ 170.29 21 Nb₂O₅ 100.58 22 SiO₂ 150.63 23 Nb₂O₅ 100.33 24 SiO₂148.03 25 Nb₂O₅ 100.94 26 SiO₂ 157.48 27 Nb₂O₅ 107.64 28 SiO₂ 147.08 29Nb₂O₅ 93.86 30 SiO₂ 154.42 31 Nb₂O₅ 91.38 32 SiO₂ 144.05 33 Nb₂O₅ 91.6834 SiO₂ 141.88 35 Nb₂O₅ 80.56 36 SiO₂ 118.17 37 Nb₂O₅ 76.74 38 SiO₂126.28 39 Nb₂O₅ 86.64 40 SiO₂ 117.78 41 Nb₂O₅ 65.51 42 SiO₂ 133.06 43Nb₂O₅ 71.01 44 SiO₂ 130.91 45 Nb₂O₅ 72.24 46 SiO₂ 119.01 47 Nb₂O₅ 72.8148 SiO₂ 105.87 49 Nb₂O₅ 63.55 50 SiO₂ 108.46 51 Nb₂O₅ 62.2 52 SiO₂ 95.5253 Nb₂O₅ 58.44 54 SiO₂ 90.83 55 Nb₂O₅ 66.17 56 SiO₂ 103.2 57 Nb₂O₅ 50.9258 SiO₂ 90.62 59 Nb₂O₅ 68.47 60 SiO₂ 89.21 61 Nb₂O₅ 50.51 62 SiO₂ 83.6663 Nb₂O₅ 52.07 64 SiO₂ 98.63 65 Nb₂O₅ 54.33 66 SiO₂ 82.56 67 Nb₂O₅ 44.9368 SiO₂ 68.87 69 Nb₂O₅ 48.9 70 SiO₂ 73.87 71 Nb₂O₅ 41.08 72 SiO₂ 74.7373 Nb₂O₅ 43.11 74 SiO₂ 74.41 75 Nb₂O₅ 44.38 76 SiO₂ 67.83 77 Nb₂O₅ 44.9678 SiO₂ 68 79 Nb₂O₅ 32.32

In this example, the second reflective layered coating can have the sameconfiguration as in Example 1. Therefore, the second reflective layeredcoating can have the configuration shown in TABLE 2.

Example 3

A broadband reflecting mirror of this example is a broadband reflectingmirror exhibiting its highest reflectance when the angle of incidencethereon is approximately 23°.

Like Example 1, Nb₂O₅ is used as each of first high-refractive indexmaterial layers, SiO₂ is used as each of first low-refractive indexmaterial layers. TABLE 4 shows the configuration of the first reflectivelayered coating.

TABLE 4 Angle of Incidence: 23° Layer No. Type Thickness (nm) 1 Nb₂O₅136.5 2 SiO₂ 208.75 3 Nb₂O₅ 137.89 4 SiO₂ 227.15 5 Nb₂O₅ 129.77 6 SiO₂203.55 7 Nb₂O₅ 120.74 8 SiO₂ 201.05 9 Nb₂O₅ 127.37 10 SiO₂ 200.48 11Nb₂O₅ 119.37 12 SiO₂ 189.84 13 Nb₂O₅ 128.63 14 SiO₂ 209.18 15 Nb₂O₅126.92 16 SiO₂ 182.94 17 Nb₂O₅ 114.19 18 SiO₂ 187.72 19 Nb₂O₅ 114.88 20SiO₂ 153.38 21 Nb₂O₅ 90.84 22 SiO₂ 148.41 23 Nb₂O₅ 95.41 24 SiO₂ 166.3225 Nb₂O₅ 103.95 26 SiO₂ 157 27 Nb₂O₅ 108.05 28 SiO₂ 144.57 29 Nb₂O₅89.52 30 SiO₂ 146.8 31 Nb₂O₅ 98.99 32 SiO₂ 150.93 33 Nb₂O₅ 103.92 34SiO₂ 138.96 35 Nb₂O₅ 75.47 36 SiO₂ 122.69 37 Nb₂O₅ 81.64 38 SiO₂ 118.3439 Nb₂O₅ 75.44 40 SiO₂ 114.62 41 Nb₂O₅ 74.31 42 SiO₂ 147.79 43 Nb₂O₅80.06 44 SiO₂ 111.53 45 Nb₂O₅ 62.86 46 SiO₂ 134.3 47 Nb₂O₅ 71.99 48 SiO₂120.28 49 Nb₂O₅ 67.91 50 SiO₂ 110.47 51 Nb₂O₅ 66.89 52 SiO₂ 99.51 53Nb₂O₅ 49.67 54 SiO₂ 93.51 55 Nb₂O₅ 69.52 56 SiO₂ 92.38 57 Nb₂O₅ 53.59 58SiO₂ 89.01 59 Nb₂O₅ 63.39 60 SiO₂ 87.74 61 Nb₂O₅ 62.19 62 SiO₂ 96.03 63Nb₂O₅ 49.4 64 SiO₂ 96.34 65 Nb₂O₅ 58.03 66 SiO₂ 79.19 67 Nb₂O₅ 41.13 68SiO₂ 73.37 69 Nb₂O₅ 44.99 70 SiO₂ 82.45 71 Nb₂O₅ 37.88 72 SiO₂ 62.32 73Nb₂O₅ 50.2 74 SiO₂ 80.21 75 Nb₂O₅ 37.7 76 SiO₂ 68.9 77 Nb₂O₅ 40.87 78SiO₂ 87.92 79 Nb₂O₅ 36.89

In this example, the second reflective layered coating can have the sameconfiguration as in Example 1. Therefore, the second reflective layeredcoating can have the configuration shown in TABLE 2.

Example 4

A broadband reflecting mirror of this example is a broadband reflectingmirror exhibiting its highest reflectance when the angle of incidencethereon is approximately 30°.

Like Example 1, Nb₂O₅ is used as each of first high-refractive indexmaterial layers, SiO₂ is used as each of first low-refractive indexmaterial layers. TABLE 5 shows the configuration of the first reflectivelayered coating.

TABLE 5 Angle of Incidence: 30° Layer No. Type Thickness (nm) 1 Nb₂O₅144.05 2 SiO₂ 225.97 3 Nb₂O₅ 122.55 4 SiO₂ 211.65 5 Nb₂O₅ 130.46 6 SiO₂214.64 7 Nb₂O₅ 120.68 8 SiO₂ 196.94 9 Nb₂O₅ 122 10 SiO₂ 211.9 11 Nb₂O₅127.72 12 SiO₂ 204.8 13 Nb₂O₅ 127.64 14 SiO₂ 204.72 15 Nb₂O₅ 128.04 16SiO₂ 186.01 17 Nb₂O₅ 117.36 18 SiO₂ 195.02 19 Nb₂O₅ 109.18 20 SiO₂160.15 21 Nb₂O₅ 93.52 22 SiO₂ 140.44 23 Nb₂O₅ 99.33 24 SiO₂ 186.99 25Nb₂O₅ 105.68 26 SiO₂ 174.45 27 Nb₂O₅ 95.77 28 SiO₂ 149.48 29 Nb₂O₅ 89.6230 SiO₂ 152.64 31 Nb₂O₅ 102.12 32 SiO₂ 152.85 33 Nb₂O₅ 96.69 34 SiO₂142.49 35 Nb₂O₅ 80.25 36 SiO₂ 119.16 37 Nb₂O₅ 88.1 38 SiO₂ 124.95 39Nb₂O₅ 60.62 40 SiO₂ 124.26 41 Nb₂O₅ 88.05 42 SiO₂ 137.48 43 Nb₂O₅ 82.1544 SiO₂ 104.8 45 Nb₂O₅ 64.73 46 SiO₂ 128.62 47 Nb₂O₅ 74.5 48 SiO₂ 129 49Nb₂O₅ 56.16 50 SiO₂ 136.23 51 Nb₂O₅ 59.62 52 SiO₂ 89.18 53 Nb₂O₅ 60.0954 SiO₂ 106.52 55 Nb₂O₅ 52.32 56 SiO₂ 101.96 57 Nb₂O₅ 59.45 58 SiO₂115.89 59 Nb₂O₅ 52.17 60 SiO₂ 84.98 61 Nb₂O₅ 51.18 62 SiO₂ 93.84 63Nb₂O₅ 69.53 64 SiO₂ 86.76 65 Nb₂O₅ 48.69 66 SiO₂ 75.22 67 Nb₂O₅ 42.29 68SiO₂ 79.18 69 Nb₂O₅ 38.91 70 SiO₂ 68.32 71 Nb₂O₅ 37.62 72 SiO₂ 75.9 73Nb₂O₅ 39.11 74 SiO₂ 90.52 75 Nb₂O₅ 30.69 76 SiO₂ 92.33 77 Nb₂O₅ 35.09 78SiO₂ 97.16 79 Nb₂O₅ 29.63

In this example, the second reflective layered coating can have the sameconfiguration as in Example 1. Therefore, the second reflective layeredcoating can have the configuration shown in TABLE 2.

Example 5

A broadband reflecting mirror of this example is a broadband reflectingmirror using germanium as each of second high-refractive index materiallayers. Since germanium has high absorption in a short wavelength bandup to 2000 nm as described previously, the first reflective layeredcoating is designed to reflect light within a wavelength band of 400 nmto 2000 nm. On the other hand, the second reflective layered coating isdesigned to reflect light within a wavelength band of 2000 nm to 2500nm.

TABLE 6 shows the configuration of the first reflective layered coatingwhen Nb₂O₅ is used as each of first high-refractive index materiallayers and SiO₂ is used as each of first low-refractive index materiallayers.

TABLE 6 Layer No. Type Thickness (nm) 1 Nb₂O₅ 204.04 2 SiO₂ 309.39 3Nb₂O₅ 197.04 4 SiO₂ 314.42 5 Nb₂O₅ 205.12 6 SiO₂ 322.33 7 Nb₂O₅ 197.5 8SiO₂ 319.66 9 Nb₂O₅ 196.1 10 SiO₂ 306.03 11 Nb₂O₅ 190.77 12 SiO₂ 296.1813 Nb₂O₅ 178.65 14 SiO₂ 284.54 15 Nb₂O₅ 180.52 16 SiO₂ 259.67 17 Nb₂O₅130.91 18 SiO₂ 223.42 19 Nb₂O₅ 141.66 20 SiO₂ 257.94 21 Nb₂O₅ 174.11 22SiO₂ 317.55 23 Nb₂O₅ 129.96 24 SiO₂ 219.77 25 Nb₂O₅ 140.53 26 SiO₂217.44 27 Nb₂O₅ 138.62 28 SiO₂ 217.3 29 Nb₂O₅ 136.83 30 SiO₂ 197.46 31Nb₂O₅ 125.34 32 SiO₂ 198.65 33 Nb₂O₅ 118.46 34 SiO₂ 199.9 35 Nb₂O₅ 124.736 SiO₂ 200.17 37 Nb₂O₅ 125.52 38 SiO₂ 178.4 39 Nb₂O₅ 115.29 40 SiO₂177.09 41 Nb₂O₅ 109.48 42 SiO₂ 171.14 43 Nb₂O₅ 102.79 44 SiO₂ 163.03 45Nb₂O₅ 98.04 46 SiO₂ 151.06 47 Nb₂O₅ 98.01 48 SiO₂ 152.7 49 Nb₂O₅ 108.1550 SiO₂ 153.73 51 Nb₂O₅ 91.45 52 SiO₂ 150.35 53 Nb₂O₅ 92.17 54 SiO₂134.35 55 Nb₂O₅ 87.96 56 SiO₂ 136.55 57 Nb₂O₅ 81.04 58 SiO₂ 127.16 59Nb₂O₅ 77.38 60 SiO₂ 129.75 61 Nb₂O₅ 83.57 62 SiO₂ 114.1 63 Nb₂O₅ 69.8664 SiO₂ 122.19 65 Nb₂O₅ 74.93 66 SiO₂ 121.13 67 Nb₂O₅ 73.43 68 SiO₂120.02 69 Nb₂O₅ 71.54 70 SiO₂ 106.34 71 Nb₂O₅ 64.53 72 SiO₂ 107.75 73Nb₂O₅ 63.63 74 SiO₂ 98.23 75 Nb₂O₅ 60.33 76 SiO₂ 90.62 77 Nb₂O₅ 65.35 78SiO₂ 93.61 79 Nb₂O₅ 51.52 80 SiO₂ 89.76 81 Nb₂O₅ 68.28 82 SiO₂ 89.59 83Nb₂O₅ 50.86 84 SiO₂ 83.07 85 Nb₂O₅ 53.37 86 SiO₂ 100.92 87 Nb₂O₅ 48.9 88SiO₂ 78.7 89 Nb₂O₅ 45.32 90 SiO₂ 72.55 91 Nb₂O₅ 45.98 92 SiO₂ 72.16 93Nb₂O₅ 45.17 94 SiO₂ 73.19 95 Nb₂O₅ 43.1 96 SiO₂ 72.64 97 Nb₂O₅ 43.37 98SiO₂ 67.11 99 Nb₂O₅ 41.08 100 SiO₂ 66.64 101 Nb₂O₅ 33.74

As shown in TABLE 6, the number of layers of the first reflectivelayered coating is 101.

TABLE 7 shows the configuration of the second reflective layered coatingwhen Ge (germanium) is used as each of second high-refractive indexmaterial layers and SiO₂ (silicon oxide) is used as each of secondlow-refractive index material layers.

TABLE 7 Layer No. Type Thickness (nm) 1 SiO₂ 50 2 Ge 74.36 3 SiO₂ 247.734 Ge 74.36 5 SiO₂ 247.73 6 Ge 74.36 7 SiO₂ 247.73 8 Ge 74.36 9 SiO₂317.09 10 Ge 116 11 SiO₂ 386.46 12 Ge 116 13 SiO₂ 386.46 14 Ge 116 15SiO₂ 386.46 16 Ge 116 17 SiO₂ 495.46 18 Ge 148.72 19 SiO₂ 495.46 20 Ge148.72 21 SiO₂ 100

As shown in TABLE 7, the number of layers of the second reflectivelayered coating is 21.

Therefore, the total number of layers in the entire broadband reflectingmirror is 122, which is smaller than 146, the number of layers when thebroadband reflecting mirror is composed only of Nb₂O₅ and SiO₂, but islarger than the number of layers in Example 1.

Example 6

A broadband reflecting mirror of this example is a broadband reflectingmirror in which a metal coating is provided as a third reflectivecoating on a second reflective layered coating. The mirror is abroadband reflecting mirror exhibiting its highest reflectance when theangle of reflection thereon is approximately 20°.

The configuration of a first reflective layered coating in this exampleis the configuration shown in TABLE 1.

The configuration of the second reflective layered coating and the thirdreflective coating in this example is the configuration shown in TABLE8.

TABLE 8 Layer No. Type Thickness (nm) 1 Si 103 2 SiO₂ 263 3 Al 200

As shown in TABLE 8, the second reflective layered coating is formed bystacking a Si layer and a SiO₂ layer. An Al layer serving as a thirdreflective coating is formed on the SiO₂ layer.

The formation of the Al layer as a third reflective coating allows thenumber of layers of the second reflective layered coating to be reducedas shown in TABLE 8.

FIG. 11 is a graph showing the reflectance of this example atwavelengths of 400 nm to 2500 nm. As is evident from FIG. 11, thebroadband reflecting mirror of this example also has achieved highreflectance over the entire wavelength band of 400 nm to 2500 nm. Inaddition, it has been confirmed that the reflectance of this exampleafter being heated at 300° C. for 264 hours is the same as in FIG. 11and the example has excellent thermal resistance.

Example 7

A broadband reflecting mirror of this example is, like Example 6, abroadband reflecting mirror in which a metal coating is provided as athird reflective coating on a second reflective layered coating. Themirror is a broadband reflecting mirror exhibiting its highestreflectance when the angle of incidence thereon is approximately 20°.

A first reflective layered coating has the configuration shown in TABLE1.

The second reflective layered coating and the third reflective coatinghas the configuration shown in TABLE 9.

TABLE 9 Layer No. Type Thickness (nm) 1 SiO₂ 50 2 Si 103 3 SiO₂ 263 4 Al200 5 SiO₂ 50

The SiO₂ layer of Layer No. 5 in TABLE 9 is a protective layer forprotecting the Al coating serving as a third reflective coating.Furthermore, the SiO₂ layer of Layer No. 1 is a layer provided forimproving the adhesion of the Si layer of Layer No. 2 to the glasssubstrate.

The second reflective layered coating is composed of Layers Nos. 1 to 3.The Si layer is a second high-refractive index material layer, and theremaining SiO₂ layer is a second low-refractive index material layer.

As shown in TABLE 9, the provision of the Al layer as a third reflectivecoating allows the number of layers of the second reflective layeredcoating to be reduced. In this example, the SiO₂ layer (Layer No. 5)serving as a protective layer is provided outside of the Al layer,whereby the thermal resistance can be further enhanced.

The reflectance of this example at wavelengths of 400 nm to 2500 nm wasas shown in FIG. 11, which was the same as in Example 6. In addition, ithas been confirmed that like Example 6 this example has excellentthermal resistance.

(Solar Concentrating System) FIG. 10 is a schematic diagram showing anexample of a solar concentrating system. In the solar concentratingsystem shown in FIG. 10, a plurality of heliostats 6 are placed on theground. Each heliostat 6 can change the angle of its reflecting mirroraccording to the position of the sun to direct the reflected light ofthe sunlight 5 to a converging point 9. In order to allow the reflectedlight from the heliostat 6 to be reflected, a light-collectingreflecting mirror 7 is provided at a high elevation short of theconverging point 9. The light-collecting reflecting mirror 7 reflectsthe reflected light from the heliostat 6 to collect it to a solarcollector 8 provided near the ground. The reflected light beams from allthe heliostats 6 are directed to the light-collecting reflecting mirror7, reflected by the light-collecting reflecting mirror 7 and thencollected by the solar collector 8.

The broadband reflecting mirror of the present invention can be used asthe reflecting mirror for each heliostat 6 or as the light-collectingreflecting mirror 7. Particularly, the light-collecting reflectingmirror 7 is heated to high temperature since the reflected light beamsfrom all the heliostats 6 are collected thereto. Therefore, thelight-collecting reflecting mirror 7 is required to have high thermalresistance. Accordingly, the broadband reflecting mirror of the presentinvention can be suitably used as such a light-collecting reflectingmirror.

REFERENCE SIGNS LIST

-   -   1 . . . broadband reflecting mirror    -   2 . . . transparent substrate    -   3 . . . first reflective layered coating    -   4 . . . second reflective layered coating    -   5 . . . sunlight    -   6 . . . heliostat    -   7 . . . light-collecting reflecting mirror    -   8 . . . solar collector    -   9 . . . converging point

1. A broadband reflecting mirror for reflecting light within awavelength band of 400 nm to 2500 nm, the broadband reflecting mirrorcomprising: a first reflective layered coating for reflecting light in ashort wavelength side of the wavelength band of 400 nm to 2500 nm, thefirst reflective layered coating including first high-refractive indexmaterial and first low-refractive index material layers alternatelystacked one on another; and a second reflective layered coating forreflecting light in a long wavelength side of the wavelength band of 400nm to 2500 nm, the second reflective layered coating including secondhigh-refractive index material and second low-refractive index materiallayers alternately stacked one on another, wherein the first reflectivelayered coating is disposed on the light-incident side of the broadbandreflecting mirror and the second reflective layered coating is disposedat a position where light having passed through the first reflectivelayered coating can be reflected, and wherein the first high-refractiveindex material layer is formed of at least one material selected fromthe group consisting of niobium oxide, titanium oxide, zirconium oxide,tantalum oxide, hafnium oxide, silicon nitride, yttrium oxide and indiumtin oxide, the first low-refractive index material layer is formed of atleast one material selected from the group consisting of silicon oxideand magnesium fluoride, the second high-refractive index material layeris formed of at least one material selected from the group consisting ofsilicon and germanium, and the second low-refractive index materiallayer is formed of at least one material selected from the groupconsisting of silicon oxide and magnesium fluoride.
 2. The broadbandreflecting mirror according to claim 1, wherein the secondhigh-refractive index material layer is formed of silicon, thewavelength band of the short wavelength side is 400 nm to 1200 nm, andthe wavelength band of the long wavelength side is 1200 nm to 2500 nm.3. The broadband reflecting mirror according to claim 1, wherein thesecond high-refractive index material layer is formed of germanium, thewavelength band of the short wavelength side is 400 nm to 2000 nm, andthe wavelength band of the long wavelength side is 2000 nm to 2500 nm.4. The broadband reflecting mirror according to claim 1, wherein thefirst reflective layered coating and the second reflective layeredcoating are provided on a transparent substrate, the first reflectivelayered coating is disposed on one surface of the transparent substrate,and the second reflective layered coating is disposed on the othersurface of the transparent substrate.
 5. The broadband reflecting mirroraccording to claim 1, wherein the first reflective layered coating andthe second reflective layered coating are provided on a transparentsubstrate, the second reflective layered coating is disposed on top ofthe transparent substrate, and the first reflective layered coating isdisposed on top of the second reflective layered coating.
 6. Thebroadband reflecting mirror according to of claim 1, wherein the firstreflective layered coating and the second reflective layered coating areprovided on a transparent substrate, the first reflective layeredcoating is disposed immediately on the transparent substrate, and thesecond reflective layered coating is disposed on the first reflectivelayered coating.
 7. The broadband reflecting mirror according to claim1, wherein a metal coating is provided as a third reflective coating ata position where light having passed through the second reflectivelayered coating can be reflected.
 8. The broadband reflecting mirroraccording to claim 1, the broadband reflecting mirror being used as areflecting mirror for a heliostat in a solar concentrating system or asa light-collecting reflecting mirror for collecting light beamsreflected by reflecting mirrors for heliostats.